The idea that states should provide a means-tested guaranteed minimum income for citizens who are unable tomeet their basic needs is widely shared and has been a central component in the evolution of social citizenshiprights in existing welfare states. However, an increasing number of activists and scholars defend the more radicaloption of establishing a universal basic income, that is, an unconditional income paid to all members of society onan individual basis without any means test or work requirement. Indeed, some political philosophers have arguedthat basic income is one of the most important reforms in the development of a just and democratic society, and iscomparable to other milestones in the history of citizenship rights, such as universal suffrage or even theabolishment of slavery. Basic income or similar ideas, such as a basic capital or a negative income tax, have beenadvanced in many versions since the 18th century in different parts of the world and under a great variety of names.However, while these were previously often isolated and disconnected initiatives, basic income has more recentlybecome the object of an increasingly cumulative research effort to shed light on the many aspects of this idea. It hasalso inspired policy developments and given rise to experiments and pilot projects in several countries.